As the mortgage market melted down in 2007 and 2008, investment bankers and traders at Credit Suisse used fraudulent bookkeeping to create an illusion that their subprime investments were still profitable, the government said Wednesday.

Instead of marking down the value of the holdings, which would have put a dent in the investment bank’s bottom line, the Credit Suisse employees simply concocted numbers that staved off the reckoning, the government said.

The Securities and Exchange Commision accused four former Credit Suisse employees of civil fraud. Three were named in related criminal charges, and two pleaded guilty to those.

The most senior of the defendants has not settled with the government. Kareem Serageldin lives in Britain, and in a Wednesday news briefing a federal prosecutor in Manhattan urged him to come to the United States to answer the charges.

Serageldin, 39, was a managing director at Credit Suisse with a big title: global head of the structured credit group in the securities department of the investment banking division. A Credit Suisse spokesman declined to comment and would not say to whom Serageldin reported.

Serageldin “believes he’s done nothing wrong,” said his attorney, James McGuire. “He had fully coooperated with all government authorities and is extremely disappointed with today’s developments.”

Another defendant, former managing director and head of hedge trading David Higgs, 41, expressed contrition as he pleaded guilty to a conspiracy count.

“I did this because I wanted to remain in good favor with my boss . . . and enhance my job performance,” he said in the text of a statement prepared for his court appearance.

The SEC said the fraud unraveled in early 2008 when senior management at Credit Suisse noticed that bonds were being carried on the firm’s books at abnormally high values.

The firm took a writedown of $2.65 billion in March 2008, about half of which involved a portfolio the defendants controlled, the SEC said.

Credit Suisse was not charged. The SEC said its decision not to charge the firm reflected its cooperation in the probe, the fact that the firm reported the problem to the SEC and “the isolated nature of the wrongdoing.”

The government’s allegations are consistent with a suspicion widely held during the financial crisis: that big financial institutions were papering over the damage and hiding its true extent from the public.

Some of the defendants were being pressured to avoid showing losses, the SEC said. Serageldin received a $1.7 million bonus for 2007 plus a share award worth more than $5.2 million, the Manhattan U.S. attorney’s office said. The share award was revoked when the fraud was uncovered, the prosecutor’s office said.

The investigation, which began in 2008, drew upon internal Credit Suisse recordings of employee conversations, much of those involving the profit and loss or “P&L” they wanted to show on a running basis.

“What sort of P&L do you need today?” a trading assistant asked Higgs on Sept. 17, 2007, according to the SEC complaint.

Higgs responded that the books needed to end the day up about $35 million, the SEC alleged.

At one point, Seragaldin expressed concern, saying, “we can’t keep picking up pennies in front of the steamroller,” the SEC said.

The Credit Suisse employees got help from another investment bank, identified only as “Dealer C,” which provided support for the bogus bond valuations a Credit Suisse employee supplied, the SEC said.

“In the space of less than one minute, the bond salesman at Dealer C sent the bonds and prices back . . . making them look like genuine third-party prices when they were not,” the SEC said.